Device for use in determining photographic exposures



July 6, 1948. D. w. NORWOOD DEVICE FOR USE IN DETERMINING PHOTOGRAPHIC EXFOSURES' 3 Sheets-Sheet 1 Filed Feb. 8, 1947 DONALD W. NORWOOD ,4 TF'QR/VEI y 1943- D. w. NORWOOD 2,444,464

. DEVICE FOR USE IN DETERMINING YHOTOGRAPHIC EXPOSURES Filed Feb. 8, 1947 3 Sheets-Sheet 2 1U INVENTOR.

DONALD -w. NORWOOD July 6, 1948. D. w. NORWOOD 2,444,464

DEVICE FOR USE IN DETERMINING PHOTOGRAPHIC EXPOSURES Filed Feb. 8, 1947 3 Sheets-Sheet 3 INVENTOR.

" DONALD W. NORWOOD Patented July 6, 1948 UNITED PATENT OFFICE .pEvIo FDR v se IN DETERMINING no'roqa PHIo- XPosUREs 3- Claim llhis invention relates to the photographicart, and pertains particularly to a device which enables the rapid and, exact calculation of the required photographic exposure under varied lighting conditions encountered in ordinary 'use. The device is intended to be used in cqnjunction with a light intensity measuring device adapted-to measure the light intensity'incident upon 'the photographic subject (rather than the intensity of the light reflected from the subjectl the'data obtained from the light-intensity measuring device being directly utilized in the ascertainment of the desired result, for example, the proper values of lens aperture and shutter speed for a givencondition of film speed and lighting.

The proper appraisal of the photographiceffectiveness of the illumination of a particular subject has alwayspresented-difiiculties which resuit in the user of any photographic exposure-estimating device having to make a mental correction of the Value indicated by such device in rder to compensate for some condition whichthedevviceitself cannot be depended upon to compensate. For. example, in the use of the reflected light meter, an assumption is made that the average scene reflects a: certain percentage bf:=the'-incident light; if the scene departs fromfthis certain reflectance, the user must compensate f or' the departure by a mental compensation'of the-result indicated by the device. This mental compensation relies upon thejudgment'of-theuser, and is subject to considerable error. -As a general rule, a fairly good result may be obtained where black-and-White film material is being employed, as a result of thelatitude such film has for exposure, but wherethe socalle'd-- naturalcolor films are employed, a much closerdeterv mination of the optimum exposure must be' attained in order that good color rendition is obtained.

I have observed that a very close determination cfthe p p h h c $99 M? 3. .45":

camera axis todeterminethe jeffectiye illumination. This latter featurev makes-itpossible to utilize the key light illumination as thoughit were the entire illumination, insofarasdetermination of the photographicexposureisconcerned.

This requires the usev of .a sorcalledfiincidentlight type-0f. exposure meter,..and the-proper .Ltc register .with the indicia of one of the exposure application of the values obtained, in the ensuing computation of the photographic exposure. -.The.device of the present invention is, adapted .to provideproper computationof the correct exposure for a photographic subject when used with a key light measuring device of the incident- .light type. .Oneof the particular objects of the .invention is to provide a device for the determination of photographic exposure which enables the user to consider the eifective angle of the illuminationprovided bythe key light.

.A further object of the invention is to provide a device for the determination of photographic .exposure, which is provided with the customary scales for the different factors to be considered, such as film sensitivity, light intensity, shutter time and lens aperture, together with indices for .establishing the photographic exposure with reference to the angle of effective illumination pro- ..yided by the key light which illuminates the photographic subject. "-The device in accordance with thisinvention comprises a plurality of exposure factor scale ...members and a key light angle scalemember interconnected for relative rotation about a com- ..mon axis. The exposure factor scale members are provided with concentric scalesindicative of different factors involved in determining photographic exposure, commonly referred to as exposure factors. Each of these scales has a plu- Z.rality of angularly spaced indicia representing ..difler.ent values of the corresponding exposure factor. Ingeneral, the exposure factors reprev.sented by thedifierent scales include light in- ..tensity values as measured by an incident light meter, film sensitivity, lens aperture or f-s'top, .and exposure time or shutter time.

The key light angle scale member is pro- .vided with areference mark that is positioned .qfactor. scales, and with avplurality of angularly .spac ed marks or indices that are positioned to [register with theindicia of another of the exposure factor scales. These angularly spaced indices represent different values of key light angle, that is, the angular relationship between .-,the.camera,,axis and the direction of the main .or fkey illumination at the position of the photographic subject.

The several scale members are adapted to be .set, by relative rotation, to indicate the proper value of oneof the exposure factors corresponding to different values of theother exposure factors anddifferent values of the key light angle.

5 am preferred embodiments of the invention, the

3 device gives an indication of properly related values of lens aperture and shutter speed corresponding to given values of light intensity, film sensitivity, and key light angle. In order to facilitate the setting and manipulation of the scale members, 1 preferably provide releasable latch means whereby one of the scale members may be latched to another member in any desired relation thereto, so that the two members thus latched together operate as a unit relative to the remaining scale member or members.

The key light angle indices are so located angularly with respect to the reference mark on the key light angle scale member as to modify the indications of one exposure factor (corresponding to given values of the other factors) in accordance with variations in the key light angle, when the corresponding key light angle index is set to register with any one of the indicia of the coacting exposure factor scale.

The above and other features of the invention will be brought out in or will be apparent from the ensuing description of preferred embodiments thereof, as illustrated in the accompanying drawings, in which:

Fig. l is a face View of one embodiment of the device, with the dials set for the solution of a given problem;

Fig. 2 is a sectional view on line 22 in Fig. 1;

Figs. 3, 4 and 5 are face views of three separate scale members of the device shown in Figs. 1 and 2;

Fig. 6 is a fragmentary section on line 6-43 in Fig. 1;

Fig. 7 is a face view of another form of device according to the invention;

Fig. 8 is a sectional view on line 88 in Fig. '7

Figs. 9, 1c and 11 are face views of three separate scale members of the device shown in Figs. '7 and 8;

Fig. 12 is a fragmentary section on line l2-|2 in Fig. '7;

Fig. 13 is a face view of another embodiment of the invention;

Fig. 14 is a sectional view on line M-M in Fig. 13; and

Figs. 15, 16 and 17 are face views of three separate scale members of the device shown in Figs. 13 and 14.

The form of device shown in Figs. 1 through 6 comprises three scale members indicated at A, K and B, interconnected for relative rotation about a common axis indicated at X--X in Fig. 2. The scale members are shown as mounted and interconnected for relative rotation by means of a base member I to which the uppermost scale member A is secured by screws 2. The base memher I is shown as provided with an annular groove or recess 3 in which the ring-shaped lowermost scale member B is rotatably disposed with itsinner peripheral edge rotatably engaging the shoulder i at the inner periphery of recess 3,. said shoulder serving to center the scale member B about the axis X-X while permitting free: rotation thereof.

Between the shoulder 4 and the central portion $3 to which scale member A is secured, the base I is provided with another annular recess 1 of less depth than recess 3, in which the ring-shaped key light angle scale member K is rotatably mounted and centered about the axis X-X by shoulder 3 in a manner comparable to the mounting of scale member B as described above.

The outer portion of scale member K overlaps the inner portion of scale member B to retain the 4 latter in position within the recess 3, while the outer portion of scale member A overlaps the inner portion of scale member K to retain it within the recess '1.

The scale members A and B are the exposure factor scale members in this form of the device. The uppermost scale member A comprises a substantially semi-circular portion A1 which is shown as being of substantially the same radius as scale member K, and a substantially semi-circular portion A2 of smaller radius, whereby approximately half the circumferential extent of the intermediate scale member K is exposed to view, as may be seen in l. The portion A1 is provided with a lens aperture scale indicated generally at l I, having a plurality of angularly spaced indicia ii in the form of printed marks and associated numbers representing the different values of lens aperture or if-stop, said aperture values increasing in a counter-clockwise direction (it being understood that a decrease in the f-stop number represents an increase in aperture value). The indicia ii are arranged peripherally of the portion A of this scale member and are shown as representing lens aperture values from f.1 to 14:5.

The portion A2 of this scale member is provided with a film sensitivity scale l2 having a plurality of angularly spaced indicia l2 arranged along the periphery thereof and comprising printed marks and associated numbers representing different values of film sensitivity according to any desired scale or system of measurement. The scale I have illustrated is based upon the Film exposure index and the American Standard speed numbers, which are identical in value and are sponsored by the American Standards Association, rather than the older Weston speed or G.-E speed scales. As illustrated in Fig. 3, this scale runs from a film sensitivity of 4 to 1600, with the values increasing in a clockwise direction.

The lowermost scale member B is provided with a shutter time scale I3 which occupies approximately one-half the circumference thereof and comprises a plurality of angularly spaced indicia l3 in the form of printed marks and associated numbers representing different values of shutter time, increasing in a clockwise direction, from ziooo of a second up to four seconds. The indicia 13 are located adjacent the inner periphery of scale member B, in a position to register with the lens aperture indicia ii of scale member A, as illustrated in Fig. l.

The scale member 13 is also provided, in the remaining portion of the circumference thereof, with a light intensity scale id comprising a plurality of angularly spaced indicia it in the form of printed marks and associated numbers representing different values of light intensity as indicated by the incident-light exposure meter with which the device is to be used, 1 preferably utilize a light scale graduated in foot-candles, arranged so that equal intervals indicate progressively increased light values according to a geometric progression. Thus, the indicia [4 are shown as representing light values from 10 to 10,000 foot-candles, with the value of light intensity increasing in a counter-clockwise direction.

To facilitate manual rotation of scale member 13, the base I may be notched or cut away at at least one position, and preferably at two opposite sides thereof, as indicated at I6, whereby the edge portion of said member may be engaged i. by a finger at either oi' thesepositions-for effectingrotation thereof.

The keylight angle scalemember K, which is located between scale members A and B; is provided with a reference mark II, locatedadjacent the inner periphery thereof in positionto register with the indicia I2" of the film sensitivity scale on scalemember A. Said scale memberK is also provided with a plurality of angularly spaced indices representing difierent values of key light angle and located along: the outer periphery thereof in position to register with the indicia it of the light intensity scale on-scale member B." The key light angleindices-are so positioned as to be useful in setting the device for different effective angles of the key light which are to be considered in arriving at the proper photographic exposure. The eifective il-' lumination angle, as above-described, is the angle between the direction of the light-source (thesun, for example, when used exte'riorly, or a strong artificial light, when used indoors) and the photographic axis, measured'fr-om the position of the subject. The subject illumination resulting from the location of the light-source very close to the camera'axis, i. e., \with the sun right behind the camera, produces what is known as front lighting, and is the most effective photographically. This lighting may be considered as 0 effective illumination angle. This 0 effective illumination angle permits a minimum setting of the camera exposure controls for a given light intensity, and the key light angle index I8 (marked O) is employed for setting the exposure calculating device for this condition. If the "key light is directed upon the subject at an angle of 90 to the camera axis, the photographic exposure should be increased 100%, and for this reason the key light angle index I9 (marked 90) is displaced from the index I8 in :a counter-clockwise directionan angular amount equal to the angular spacing of one photographic stop on the scale I I, (as, for example, the angular spacing :between 12.8 and 4) llf the effective angle of illumination is about 45, values midway between the values for 0 and 90 effective angle are required; The key light angle index 20 for the"45 illumination angle is thus placed midway between the indices 58 and I9. Similarly, in case of back lighting, illumination at an angle of 135 to the subjectcamera axis requires double the exposure required for the 90 illumination, and the 135 key light angle index 2| is spaced counterclockwise from the index l9' by an angle equal to one full stop on the scale I I. It will be appreciated that if the effective angle of the illumination is not exactly at 0, "45 or 90 degree, etc., the estimated angle may be employed by approximating the'setting of the scale members at any intermediate value, by eye, within an accuracy of less than stop, whichis adequate even for natural color films.

It will be observed that scale member K may be rotated'so as to bring the reference mark I'I into register .With any one ofthe indicia I2 of the film sens-itivity'scale I2; In order to facilitate the setting and manipulation of the scale members I prefer to provide means for releasably securing or latching scale member K in any such position of adjustment, relative to the base I and the scale member A. For this purpose, I have shown a spring latch member 2 8 located in a radially extending groove 21 in the base membar I; said latch being secured cent'rally to said base memberthroughlthe agency of a screw 28 and carryinga pin 29' adapted to. engage in any one of a series of openings. 3! in scale member K, said openings being spacedangularly to-correspond with the angular spacing of the indicia of scale I2. The latch member 26 is shown as extending-beyond the periphery of scale member B and provided at its outer end with anupstanding portion 3I- adapted to be engaged by the finger of the user and depressed to disengage the pinZS from any one of the openings 5%, follow ing which the scale member'K vmay be rotated by manual engagement, to provide the desired setting of reference mark ll with respect to scale I2. The latching member 26 may then be released to latch the scale member K in this position.

In Fig. 1, the scale member K is shown as set for a film sensitivity of 100. To make this adjustment, the latch pin 29 is disengaged as above described, member K is rotated to bring reference mark i! into register with the number. 100 of the film sensitivity scale it, and latch pin 29 is then engaged with the corresponding opening 38.

Let it be assumed that the incident-light meter with which the device is used has given a reading of 320 foot-candles for the key light intensity, and that the key light angle is 45. With the member K latched in the position described above, the scale member B is then rotated to bring the 320 graduation of the light intensity scale it into register with 45 key light angle index. mark 20 on scale member K, as indicated in Fig. 1, so as to indicate on scales II and I3 the proper exposure control settings for the above assumed conditions of film sensitivity, light intensity and key light angle. Thus, the proper value of lens aperture may be read on scale I! opposite any desired value of shutter time on Scale I3.

It will be seen that the indicated exposure con-- trol settings are, for example:

second at f.2.8, or I /50 second at f.4, or second at f.5.6, etc.

If the key light angle were, for example, 0 or the 320 mark on scale it would be set opposite the corresponding index mark 18 or 19 of the key light angle scale on member K, and the proper exposure control settings for either of these conditions Would be indicated on scales II and I3, the proper aperture values being, respectively, one half-stop less or one half-stop greater than for the 45 key light angle, for each value of shutter time.

It will be obvious that the correlation between the angular spacings of the indicia of the several scales and the. location of reference mark l1 relative to the key light angle indices 8 to El are such as to provide for similar modification of-the indications of the photographic exposure in accordance with variations in the key light angle, for any other conditions of film sensitivity and light intensity, when the corresponding one of the lightintensity. indicia is set to register with the proper key light angle index. For example, if the measured value of light intensity were twice as great as assumed above, the 640 mark on scale It would be set opposite the corresponding key light angle index mark, and the indicated photographic exposure would in each case be half asinthe first described example.

Various modifications can be made in the ar rangement ofthe scales in the above described form of the invention. For example, the shutter time scale I3 and the light intensity scale I4 may be interchanged in position on the scale member B and reversed in direction of increasing values, with the shutter time scale coacting with the indices I8 to 2I of the key light angle scale on member K at the left side of Fig. l, and the light intensity scale coacting with the lens aperture scale II of member A at the right side of Fig. 1. The operation of the device with this scale arrangement would be the same as described above except that, in the final setting, a selected value of shutter time would be set (by rotation of scale member B) opposite the proper key light angle index at the left side of Fig. 1, and the proper lens aperture value would be read opposite the measured value of light intensity at the right side of Fig. 1.

As another modification, the lens aperture scale II and light intensity scale I4 may be interchanged in position, without change in the direction of increasing values thereof. In that case, the operation would again be the same as described above with the scale arrangement shown in Figs. 1 to 6, except that in the final set-- ting the scale member B would be rotated to set a selected shutter time value opposite the measured value of light intensity at the right side of Fig. 1, and the desired lens aperture value would be read opposite the proper key angle index at the left side of the device. Alternatively, a selected lens aperture value could be set opposite the key light angle index, and the proper shutter time could be read opposite the measured light intensity value.

As another alternative, the shutter time scale I3 could be substituted in place of the film sensi tivity scale I2 on member A, with the values increasing in the same direction as shown, the film sensitivity scale I2 substituted in place of the light intensity scale I4 on member B and reversed in direction of increasing values, and the light intensity scale I4 substituted in place of the shutter time scale I3 on member B and reversed in direction of increasing values. With this scale arrangement, the scale member K may be rotated to set the reference mark 5'! opposite a selected shutter time at the left side of Fig. 1, and latched in this position. Scale member B may then be rotated to set the film sensitivity value opposite the proper key light angle index at the left side of the device, and the proper lens aperture value may be read opposite the measured value of light intensity at the right side of the device.

Still another modification is to place the key light angle indices I8 to 2I adjacent the inner periphery of scale member K so as to register with the film sensitivity scale I2, and reverse the relative angular position of the key light angle index values, and place the reference mark ii at the outer periphery of scale member K so as to register with the light intensity scale i l. In this case, scale member K would be rotated so as to set the proper key light angle index mark opposite the film sensitivity value on scale I2, and latched in that position. The user may then rotate scale member B to set the light intensity value on scale I4 opposite the reference mark I1, and read the lens aperture value for any given value of shutter time on the scales II and I3.

Various modifications can also be made in the construction of the device according to my invention. For example, the intermediate scale member K could be rigidly secured to the base I and the uppermost scale member A rotatably mounted thereon and provided with releasable latch means comparable to the latch means described above. With the various scales and the reference mark I'I located on the respective scale members in the same manner as shown in Figs. 1 to 6, the user would first rotate scale member A to bring the proper value of film sensitivity on scale I2 into register with the reference mark i7, and latch said scale member in this position relative to the member K. The operation would be otherwise the same as previously described, and it will be obvious that other scale arrangements comparable to those mentioned above may also be used with this modified construction.

Another possible variation is to adjustably latch two of the scale members together for concurrent rotation relative to the other scale member which may be secured to the base. An example of such a construction. is shown in Figs. '7 through 12. The scale members A, K and B are generally similar to members A, K and B, scale member A being similarly secured to a base member I, and scale members B and K being similarly mounted in recesses 3 and I for rotation relative to member A, and being provided with in-terengaging releasable latch means.

The arrangement of the scales could be the same as in Fig. 1, but I have shown a modified scale arrangement in which a film sensitivity scale comparable to scale I2 of Fig. 1 but reversed in direction of increasing values is provided at the left side of scale member B as indicated at 3-3 (in the position occupied by the light intensity scale I4 in Fig. 1), while a light intensity scale comparable to said scale I4 but reversed in direction of increasing values is provided on the smaller radius portion of scale member A as indicated at 34 (in the position occupied by the film sensitivity scale I2 in Fig. 1).

The scale member K is provided with reference mark I! and key light angle indices I8, I9, 20 and 2I in the same manner as described above for the member K in Fig. 1, the mark I! being in this case positioned to register with any one of the light intensity indicia 34 of scale 34, and the key light angle indices l8 to 2I being positioned to register selectively with any one of the film sensitivity indicia 33 of scale 33. The scale members A and B are provided, as before, with a lens aperture scale II and a shutter time scale I3 similar to the corresponding scales in Fig. l.

The means for releasably latching together scale members K and B is shown as comprising a small, preferably somewhat rounded, downwardly projecting detent 36 on member K adapted to engage any one of a plurality of coacting recesses or openings 31 in member B located at intervals corresponding to the angular spacing between the film sensitivity indicia 33, there being one of said recesses or openings 3'! in position for engagement by detent 36 when any one of the key light angle indices I8 to 2! is set to register with any one of the indicia 33. The ring-shaped member B extends inwardly beneath the member K sufficiently to permit location of the recesses 31 at the proper position for engagement by detent 36.

It will be understood that in any one of the forms of device described herein, the several scale members may be made of thin metal or any other suitable material, and in this form of construction the member K may be sufiiciently thin and resiliently flexible to enable the detent 36 to pass readily'into and out of engagement with successive recesses Bl upon manualrotation of one of the members B or K while the other is held against rotation as by manual pressure thereon, thus permitting relative rotation of said members to establish ,a given setting of film sensitivity and key light angle. To facilitate this operation, the member K may be provided with a projecting tab 38 for manual engagement, and said tab is preferably located adjacent the detent 35 whereby a slight manual upward pressure may be applied against said tab to assist in releasing said detent from engagement with recesses 37. However, if both of the members K and F are left free to rotate, the engagement of said detent with a given recess is suificient to cause said members to rotate together, relative to member A, upon manual rotation of one of them.

In the preferred method of utilizing this form of calculating device, the scale member B is first rotated relative to scale member K, as described above, to brim the proper film sensitivity value on scale 53 into register with the desired one of the key light angle indices 48 to 2!. Scale members B and K are then rotated together, as described above, to bring the reference mark ll into register with the measured value of the key light on the light intensity scale 3d, whereupon the proper photographic exposure, in terms of properly related values of lens aperture and shutter speed, may be read on scales H and H3.

The scale settings and indicated values of lens aperture and shutter speed shown. in this case are the same as in Fig. 1.

It will be obvious that the positions of the various scales may also be interchanged in this form of the device, with due regard to the direction of increasing values thereof, in manners comparable to the modifications described in connection with the form of device shown in Fig. 1. 7

By way of example, one modificationthat may be made in the arrangement of the scales in this form or device, is to interchange the positions of the lens aperture scale H and the light intensity scale 3% on scale member A, and reverse the direction of increasing values in each of these scales. rangement, the device may be operated in the same manner as described above for the scale arrangement shown in Fig. 7, except that in the final setting operation the scale members B and K are rotated together to set the reference mark ii on member K opposite a selected lens aperture value at the left side of Fig. 7, and the proper shutter time is then read on scale it opposite the measured value of light intensity at the right side of Fig. 7. Alternatively, a selected shutter time may be set opposite the measured value of light intensity at the right side of Fig. '7, and the corresponding lens aperture value may be read opposite the reference mark H at the left side of the device.

Another modified form of device according to this invention is shown in Figs. 13 through 17. This device comprises two exposure factor scale members A and B" and the key light angle scale member K" mounted on a base member i and interconnected for relative rotation by means of a centrally located stud il secured to said base member by screw d2.

The base member is provided with an annular recess 43in which the lowermost scale member With this modified scale ar-.

10 B" is rotatably mountedin a. manner comparable to the. mounting. of the member B in Fig. 1.

Scalemember A" is'formed as a disc overlying the central portion of the base member with the outer portion thereof overlapping the ring shaped scale member'B" to retain the latter in the recess 43, scalemember A being secured to base I against rotation relative thereto, as by means of apin '44 secured to said base and engaging suitably positioned opening M insaid scale member.

Scale member K" is rotatably disposed above the scale member A" and the outer end thereof conforms substantially to the circumference of member A. Said scalememberK" however is of limited circumferential extent so asto cover only a small portion of scale member A to which it may be releasablylatched as describedhereinafter.

Scale member A" is shown as provided at the right sidethereof with theshutter time scale 45 provided with indicia 45,,said, scale being comparable to the scale [3 in Fig. .1 but with the values of shutter time increasing in a counter-clockwise direction andcovering a somewhat smaller range from 1 3 of a second to two seconds. This scale is located along the periphery of said scale member. At the opposite side, said, scale member A" is provided with afilm sensitivity scale it comprising. a plurality ofangularly spaced indiciaflt in the form of printed numbers representing different values of film .sensitivity or Film exposure index, increasing in aclockwise direction, from 3 to 1000. The indicia .46 are arranged in a circular row and positioned somewhat inwardly from the, periphery .of thescale member.

The scale member'B" is provided at the right side with'the'lens aperture scale 48 havi a pluralityofiangularly. spaced indicia 48' in position to register with the. shutter time indicia 45' on member A". This scale'48 is comparable toscale I I in'Fig. 1, but with the'lens aperture values increasing in a clockwise direction. Atthe opposite side, said scale member B" is providedwith a lightintensityzscale 49 having aplurality of indicia 49" in position to register with the key light angle indices at the outer end of scale member K", as described hereinafter. The scale ii! is similar tothe light intensity scale i4 in'Fig. 1.

The key light angle scale member;K" is pro- Vided with a reference mark in the formv of an opening 5! located inward from the periphery thereof at a position corresponding to the position of the film sensitivity indicia 46 and adapted to register with any one of said indicia upon rotation of said scale member K" to a position to permit the desired value of film sensitivity to be viewed throughsaid opening and said scale member is also provided at its-outer end with a plurality of angularly spaced indices I8, :9, 2i! and 2| which are similar in location and spacing to the correspondingly numbered indices in Fig. 1. Said scale member K" is also provided with a small downwardly projecting detent 53 adapted to engage any one of a plurality of angularly spaced'recesses or openings 54 in scale member A", said recesses correspondingin number and spacing to the indicia -46 of the film sensitivity scale 16. The scale'member'K is also shown as provided with an upwardly projecting tab 55 to facilitate manual rotation thereof.

It will be evident that scale member K" may be rotated by manual engagement of said tab 55, to bring the opening 5| into register with any one ofthe film sensitivityindicia 46' on scale mem- 11 her A", and the detent 53 will engage the corresponding recess 54 to latch these members together in this relative position with respect to each other.

In Fig. 13, the scale members K and A" are shown as set, by relative rotation in the manner described above, for a film sensitivity of 100. The scale member B" may then be rotated relative to members A" and K to set the measured value of light intensity on scale 49 opposite the proper key light angle index on member K", whereupon the proper value of lens aperture may be read on scale 48 opposite any desired value of shutter time on scale 45. The scale settings shown in Fig. 13 are identical with those in Fig. 1.

It will be evident from the above description that various modifications may also be made in the relative arrangement of the scales in this lastdescribed form of the invention, with due regard to the relative angular positions of the indicia thereof and the direction of increasing values represented by said indicia.

In each of the forms of the invention shown and described herein, and with any of the de scribed scale arrangements, it will be seen that the key light angle scale member is provided with a reference mark such as 17 or 51 positioned to register with the indicia of a coacting scale on one of the exposure factor scale members and is also provided with a plurality of angularly spaced indices, such as the marks 18 to 21, positioned to register with the indicia of a coacting scale on another exposure factor scale member, and that the key light angle indices are so located with respect to the reference mark as to modify the indicated value of one of the exposure factors corresponding to given values of the other factors, in accordance with variations in the key light angle, when the corresponding key light angle index is set to register with any of the indicia of the coacting scale on said other exposure factor scale member.

I claim:

1. In a device for calculating photographic exposure, the combination comprising: a plurality of exposure factor scale members and a key light angle scale member interconnected for relative rotation about a common axis; said exposure factor scale members being provided with concentric scales indicative of different exposure factors, including a scale indicative of intensity of the main or key illumination falling upon a photographic subject, each of said exposure factor scales having a plurality of angularly spaced indicia representing different values of the corresponding exposure factor; said key light angle scale member being provided with a reference mark positioned to register with the indicia of a coacting scale on one of said exposure factor scale members, and being also provided with a plurality of angularly spaced indices positioned to register with the indicia of a coacting scale of another of said exposure factor scale members and representing different angular relationships between the camera axis and the direction of the main or key illumination at the position of the photographic subject; said scale members being adapted to be set, by relative rotation, to indicate the proper value of one of said exposure factors corresponding to given values of the other exposure factors and given values of said angular relationship; and the indices on said key light angle scale member being so located with respect to said reference mark as to modify the indication of said one exposure factor in accordance with variations in said angular relationship, when the corresponding one of said indices is set to register with any of said indicia of said coacting scale on said other exposure factor scale member.

2. In a device for calculating photographic exposure, the combination comprising: a plurality of exposure factor scale members and a key light angle scale member interconnected for relative rotation about a common axis; said exposure factor scale members being provided with concentric scales indicative of the factors lens aperture, shutter time, film sensitivity, and light intensity, each of said exposure factor scales having a plurality of angularly spaced indicia representing different values of the corresponding factor; said key light angle scale member being provided with a reference mark positioned to register with the indicia of a coacting scale on one of said exposure factor scale members, and being also provided with a plurality of angularly spaced indices positioned to register with the indicia of a coacting scale on another of said exposure factor scale members and representing different angular rclationships between the camera axis and the di rection of the main or key illumination at the position of the photographic subject; said scale members being adapted to be set, by relative rotation, to indicate properly related values of lens aperture and shutter time corresponding to given values of film sensitivity, light intensity and said angular relationship; and the indices on said key light angle scale member being so located with respect to said reference mark as to modify the indications of related values of lens aperture and shutter time in accordance with variations in said angular relationship, when the corresponding one of said indices is set to register with any of se indicia of said coacting scale on said other (Zn.- posure factor scale member.

3. In a device for calculating photographic exposure, the combination comprising: a plurality of exposure factor scale members and a key l1ght angle scale member interconnected for relative rotation about a common axis; said exposure famtor scale members being provided with concentric scales indicative of the factors lens aperture, shutter time, film sensitivity, and light intensity, each of said exposure factor scales having a plurality of angularly spaced indicia representing; different values of the corresponding factor; key light angle scale member being provided with a plurality of angularly spaced indices positioned to register with the indicia of the scale indicative of light intensity on one of said exposure factor scale members and representing different angular relationships between the camera axis and the direction of the main or key illumination at the position of the photographic subject, and being also provided with a reference mark positioned to register with the indicia of a coacting scale on another of said exposure factor scale members; said scale members being adapted to be set, by relative rotation, to indicate properly; related values of lens aperture and shutter time corresponding to given values of film sensitivity, light intensity and said angular relationship; and the indices on said key light angle scale Iilelllber being so located with respect to said reference mark as to modify the indications of related values of lens aperture and shutter time in accordance with variations in said angular relationship, when the corresponding one of said indices is set to register with any of said indicia of said scale indicative of light intensity.

DONALD W. NORWOOD. 

